Conventionally, there is known for production of a single crystal, a pulling method called Czochralski method (CZ method) wherein a seed crystal is immersed in a melt of polycrystal raw material and is then pulled to grow a single crystal. In the method, a seed crystal held by a seed crystal holding jig needs to be strong enough to bear the weight of a grown crystal.
Recently, a single crystal, for example, CZ silicon single crystal has been getting heavier with increase in a diameter thereof. For such a tendency of increase in weight of the crystal, there have been proposed an improvement in resistance under load of the apparatus, a necking with a large diameter, or a new method substituted for a necking method, a method of holding a crystal mechanically or the like. However, strength of the crystal itself is one of important factors in common.
There have been various propositions as for a seed crystal and a seed crystal holding jig. For example, there is proposed in Japanese Patent Publication (kokoku) No. 5-36395 a seed crystal holding jig consisting of a body of a seed crystal holding jig with a taper, a piece for pressing a seed crystal and a ring for preventing the piece from dropping off. The seed crystal holding jig holds a seed crystal having a tapered cut to pull a single crystal. With such a structure, it is possible to hold a seed crystal without lowering strength of the seed crystal.
A seed crystal to be held by such a seed crystal holding jig is generally a prismatic seed crystal having a tapered cut 1 as shown in FIG. 2A, or a columnar seed crystal having a tapered cut 1 as shown in FIG. 2B. In order to pull the crystal without conducting necking, a seed crystal having a sharp tip end as shown in FIG. 2C can be used in some cases (See Japanese Patent Application No. 9-17687).
However, even when a seed crystal is held by the above-mentioned seed crystal holding jig, it is sometimes broken depending on a shape of a seed crystal or weight of a single crystal to be grown and pulled. Breakage of the seed crystal may lead to fall of the single crystal having a heavy weight in a melt in a crucible, which will be a serious accident, and is problematic. On the other hand, it is not desirable for production of a seed crystal and a necking process that a seed crystal is unnecessarily thick.
There has not been an appropriate standard for defining a shape such as a thickness of a seed crystal so far, and a seed crystal having a shape defined experientially has been used for producing a single crystal.